Patterned marking of multilayer optical film by thermal conduction

ABSTRACT

A multilayer optical film has a packet of microlayers that selectively reflect light by constructive or destructive interference to provide a first reflective characteristic. At least some of the microlayers are birefringent. A stabilizing layer attaches to and covers the microlayer packet proximate an outer exposed surface of the film. Heating element(s) can physically contact the film to deliver heat to the packet through the stabilizing layer by thermal conduction, at altered region(s) of the film, such that the first reflective characteristic changes to an altered reflective characteristic in the altered region(s) to pattern the film. The stabilizing layer provides sufficient heat conduction to allow heat from the heating elements to change (e.g. reduce) the birefringence of the birefringent microlayers disposed near the outer exposed surface in the altered region(s), while providing sufficient mechanical support to avoid substantial layer distortion of the microlayers near the outer exposed surface in the altered region(s).

FIELD

This disclosure relates to, among other things, multilayer optical filmswhose reflection and transmission characteristics are determined inlarge part by constructive or destructive interference of lightreflected from interfaces within a stack of microlayers, and techniquesfor marking such films by delivering heat to selected portions of thefilm by thermal conduction from a hot object to reduce the birefringenceof some of the microlayers, the reduction in birefringence changing thereflective and transmissive properties of the film at such locations.The disclosure also relates to articles and systems incorporating suchoptical films, and methods of making and using such films.

BACKGROUND

Multilayer optical films are known. Such films can incorporate a largenumber of thin layers of different light transmissive materials, thelayers being referred to as microlayers because they are thin enough sothat the reflection and transmission characteristics of the optical filmin or near the visible spectrum are determined in large part byconstructive and destructive interference of light reflected from thelayer interfaces. Depending on the amount of birefringence (if any)exhibited by the individual microlayers, and the relative refractiveindex differences for adjacent microlayers, and also on other designcharacteristics, the multilayer optical films can be made to havereflection and transmission properties that may be characterized as areflective polarizer in some cases, and as a mirror in other cases, forexample. Whether the reflective characteristic is a polarizer or mirror,it is also known to select the thicknesses of the microlayers so thatreflections occur in a desired part of the electromagnetic spectrum,e.g., in the visible or near infrared portion of the spectrum, or inportions thereof.

Some multilayer optical films are designed for narrow band operation,i.e., over a narrow range of wavelengths, while others are designed foruse over a broad wavelength range such as substantially the entirevisible or photopic spectrum, or the visible or photopic wavelengthrange together with near infrared wavelengths, for example. In abroadband reflector, the microlayers are arranged in optical repeatunits whose optical thickness values increase along a thickness axisfrom a first side to a second side of the film. This arrangement oflayer thicknesses is referred to as a graded layer thickness profile.

Researchers from 3M Company have pointed out the significance oflayer-to-layer refractive index characteristics of such films along thedirection perpendicular to the film, i.e., the z-axis, and shown howthese characteristics play an important role in the reflectivity andtransmission of the films at oblique angles of incidence. See, e.g.,U.S. Pat. No. 5,882,774 (Jonza et al.). Jonza et al. teach, among otherthings, how a z-axis mismatch in refractive index between adjacentmicrolayers, more briefly termed the z-index mismatch or Δnz, can betailored to allow the construction of multilayer stacks for which theBrewster angle—the angle at which reflectance of p-polarized light at aninterface goes to zero—is very large or is nonexistent. This in turnallows for the construction of multilayer mirrors and polarizers whoseinterfacial reflectivity for p-polarized light decreases slowly withincreasing angle of incidence, or is independent of angle of incidence,or increases with angle of incidence away from the normal direction. Asa result, multilayer films having high reflectivity for both s- andp-polarized light for any incident direction in the case of mirrors, andfor the selected direction in the case of polarizers, over a widebandwidth, can be achieved.

Microlayers that are birefringent can be used in mirrors, polarizers,and other multilayer optical films. Researchers from 3M Company haverecently disclosed techniques in which the reflective characteristic ofsuch a film can be pattern-wise changed by exposing the film to asuitable light beam, where energy from the light beam is used toabsorptively heat birefringent microlayers sufficiently to produce arelaxation in the material that reduces or eliminates a preexistingoptical birefringence, but low enough to maintain the layer structure ofat least most of the affected microlayers within the film. The reductionin birefringence may be partial or it may be complete, in which casesome of the interior microlayers that are birefringent in a first(untreated) zone are rendered optically isotropic in a second (treated)zone. The selective heating may be achieved at least in part byselective delivery of light or other radiant energy to the second zoneof the film. See, e.g., patent application publications WO 2010/075357(Merrill et al.), WO 2010/075340 (Merrill et al.), WO 2010/075373(Merrill et al.), WO 2010/075363 (Merrill et al.), and WO 2010/075383(Merrill et al.).

BRIEF SUMMARY

We have found that multilayer optical films that include birefringentmicrolayers can be designed to be pattern-wise marked using thermalconduction from a heated body that contacts an outer surface of thefilm. Such pattern-wise heating can be simply and conveniently carriedout, for example, using a resistive thermal printer in which an extendedheating assembly with individually addressable heating elements makessliding contact with the multilayer optical film as the film passes theheating assembly. In this regard, the contact between the film and theheated body is typically characterized by only small amounts of force orpressure, if any, in contrast to embossing techniques that use a heatedembossing tool. By appropriate control of the heating elements (e.g.turning them on and off as needed) and the speed of the film past theheating assembly, heat can be delivered to the film by heat conductionto produce a desired spatial pattern. In regions of the film to whichheat is delivered, a relaxation or elimination of birefringence in someof the microlayers produces a reflective characteristic that is modifiedor altered relative to an original or unaltered reflectivecharacteristic in the remaining (unaltered) regions of the film.

We have also found that in order to carry out the pattern-wise thermalmarking of the film, the film is desirably provided with a stabilizinglayer which attaches to a packet of the microlayers at or near the outerexposed surface of the film through which heat is conducted. Thestabilizing layer typically comprises a thermoset material, in contrastwith the microlayers, which typically comprise thermoplastic materials.The stabilizing layer is thin enough to provide sufficient heatconduction to allow heat from the heating elements to reduce thebirefringence of the birefringent microlayers disposed near the outerexposed surface in the altered regions, but the stabilizing layer isalso thick enough to provide sufficient mechanical support to avoidsubstantial layer distortion of the microlayers near the outer exposedsurface in the altered regions, which layer distortion may be manifestedas optical haze. The stabilizing layer may be used to ensure the opticalhaze in the altered region(s) does not exceed 20%, or does not exceed10%.

We have also found that the multilayer optical film can be provided witha thermal buffer layer that wholly or partially covers the stabilizinglayer. The thermal buffer layer can be used to prevent further thermalmarking after an initial thermal marking procedure, e.g., to preventmodifying or otherwise tampering with a patterned film by unauthorizedusers. In some cases the thermal buffer layer can be coated or printedin a pattern to define non-writable zones, where the thermal bufferlayer is thick, and writable zones, where the thermal buffer layer isthinner or absent.

The present application thus discloses, among other things, multilayeroptical films having a packet of microlayers that selectively reflectlight by constructive or destructive interference to provide a firstreflective characteristic. At least some of the microlayers arebirefringent. A stabilizing layer attaches to and covers the microlayerpacket proximate an outer exposed surface of the film. Heatingelement(s) can physically contact the film to deliver heat to the packetthrough the stabilizing layer by thermal conduction, at alteredregion(s) of the film, such that the first reflective characteristicchanges to an altered reflective characteristic in the altered region(s)to pattern the film. The stabilizing layer provides sufficient heatconduction to allow heat from the heating elements to change thebirefringence of the birefringent microlayers disposed near the outerexposed surface in the altered region(s), while providing sufficientmechanical support to avoid substantial layer distortion of themicrolayers near the outer exposed surface in the altered region(s).

Also disclosed are methods of making a patterned multilayer opticalfilm. Such a method may include providing a multilayer optical filmhaving an outer exposed surface and a packet of microlayers arranged toselectively reflect light by constructive or destructive interference toprovide a first reflective characteristic, at least some of themicrolayers being birefringent, the multilayer optical film alsoincluding a stabilizing layer attached to and covering the packet ofmicrolayers proximate the outer exposed surface. The method may furtherinclude physically contacting the multilayer optical film with one ormore heating elements to deliver heat at one or more altered regions ofthe film to the packet of microlayers through the stabilizing layer bythermal conduction, such that the first reflective characteristicchanges to an altered reflective characteristic in the altered regionsto pattern the multilayer optical film, the stabilizing layer beingtailored to provide sufficient heat conduction to allow heat from theheating elements to reduce the birefringence of the birefringentmicrolayers disposed near the outer exposed surface in the alteredregions, while also providing sufficient mechanical support to avoidsubstantial layer distortion of the microlayers near the outer exposedsurface in the altered regions.

The stabilizing layer may be tailored such that after the physicallycontacting, the optical haze of the optical film due to layer distortionin the altered regions is no more than 20%, or no more than 10%. Afterthe physically contacting, the patterned multilayer optical film mayhave one or more unaltered regions in addition to the one or morealtered regions, and a group of first microlayers from the birefringentmicrolayers may have respective refractive indices that aresubstantially unchanged in the altered regions relative to the unalteredregions, and a group of second microlayers from the birefringentmicrolayers may have respective refractive indices that aresubstantially changed in the altered regions relative to the unalteredregions, the group of second microlayers being closer than the group offirst microlayers to the outer exposed surface. The physical contact maybe a sliding contact, and the multilayer optical film may furtherinclude a lubricant layer comprising a non-polymer lubricant material,e.g. a wax, covering the stabilizing layer. The one or more heatingelements may include a set of individually addressable heating elements,and the method may further include providing an extended heatingassembly, the heating assembly including the individually addressableheating elements, wherein the physically contacting includes moving themultilayer optical film in relation to the extended heating assemblysuch that the outer exposed surface of the multilayer optical film makessliding contact with the heating assembly, and selectively heating theheating elements during the moving to provide the one or more alteredregions.

The method may also include, after the physically contacting is carriedout to provide the patterned multilayer optical film, coating at least afirst zone of the patterned multilayer optical film at its outer exposedsurface with a thermal buffer layer, the thermal buffer layer forming anew outer exposed surface to provide a coated patterned multilayeroptical film. The thermal buffer layer may have a sufficient thicknessso that the one or more heating elements provide little or no change inthe first reflective characteristic in the first zone of the multilayeroptical film upon physically contacting the new outer exposed surface atsuch first portion with the one or more heating elements, such that thefirst zone is a non-writable zone. The outer exposed surface may be afirst outer exposed surface and the multilayer optical film may furtherinclude a second outer exposed surface opposite the first outer exposedsurface, and the physically contacting may include physically contactingthe first outer exposed surface with the one or more heating elements toprovide one or more first altered regions, and the physically contactingmay further include physically contacting the second outer exposedsurface with the one or more heating elements to provide one or moresecond altered regions. The packet of microlayers may be characterizedby a layer thickness gradient such that microlayers proximate the firstouter exposed surface are thicker than microlayers proximate the secondouter exposed surface, such that the one or more first altered regionshave a first altered reflective characteristic and the one or moresecond altered regions have a second altered reflective characteristicdifferent from the first altered reflective characteristic.

Also disclosed are patterned multilayer optical films that have an outerexposed surface, such a film also including a packet of microlayers anda stabilizing layer. The packet of microlayers is arranged toselectively reflect light by constructive or destructive interference toprovide a first reflective characteristic, the microlayers comprisingthermoplastic materials. The stabilizing layer is attached to and coversthe packet of microlayers proximate the outer exposed surface, thestabilizing layer comprising a thermoset material. The packet ofmicrolayers may be selectively altered in a pattern to provide the firstreflective characteristic in one or more unaltered regions and a secondreflective characteristic, different from the first reflectivecharacteristic, in one or more altered regions. The packet ofmicrolayers may include a first and second group of microlayers eachhaving a birefringence in the unaltered regions, and the first group ofmicrolayers may substantially maintain the birefringence in the alteredregions, and the second group of microlayers may have a changedbirefringence in the altered regions relative to the unaltered regions,the group of second microlayers being closer than the group of firstmicrolayers to the outer exposed surface. The one or more alteredregions may have an optical haze of no more than 20%, or no more than10%.

The stabilizing layer may have a physical thickness in a range fromgreater than 0.5 microns to less than 10 microns. The stabilizing layermay be a hard coat layer. The film may further include a lubricant layerattached to and covering the stabilizing layer, the lubricant layercomprising a non-polymer lubricant material, e.g., a material thatincludes wax. The film may further include a thermal buffer layer atleast partially covering the stabilizing layer, the thermal buffer layerbeing effective to inhibit heat-induced birefringence reduction of thesecond group of microlayers in one or more zones of the film in whichthe thermal buffer layer covers the stabilizing layer, such zonesreferred to as non-writable zones. The thermal buffer layer may coversubstantially an entire major surface of the stabilizing layer, suchthat substantially all of the film is rendered non-writable. The thermalbuffer layer may alternatively be substantially absent from one or morezones of the film, such zones referred to as writable zones, such thatthe film comprises both writable zones and non-writable zones. The oneor more non-writable zones may at least partially overlap with the oneor more altered regions.

Also disclosed are multilayer optical films that have an outer exposedsurface, such a film also including a packet of microlayers and astabilizing layer. The packet of microlayers is arranged to selectivelyreflect light by constructive or destructive interference to provide afirst reflective characteristic, the microlayers comprisingthermoplastic materials, at least some of the microlayers beingbirefringent. The stabilizing layer is attached to and covers the packetof microlayers proximate the outer exposed surface. The stabilizinglayer comprises a thermoset material and is tailored to, upon exposureof a region of the film to a resistive thermal printer at the outerexposed surface, provide sufficient heat conduction to allow heat fromthe printer to change the birefringence of the birefringent microlayersdisposed near the outer exposed surface in such exposed region, whilealso providing sufficient mechanical support to inhibit distortion ofthe microlayers near the outer exposed surface in such exposed region,the changed birefringence associated with an altered reflectivecharacteristic for the packet of microlayers different from the firstreflective characteristic.

The stabilizing layer may have a physical thickness in a range fromgreater than 0.5 microns to less than 10 microns. The outer exposedsurface may be a surface of the stabilizing layer. The stabilizing layermay provide sufficient mechanical support such that, upon the exposureof the region of the film to the resistive thermal printer at the outerexposed surface, an optical haze of the optical film due to layerdistortion in such exposed region is no more than 20%, or no more than10%. The film may also include a lubricant layer covering thestabilizing layer, the lubricant layer comprising a non-polymerlubricant material, e.g., a material that includes wax. The stabilizinglayer may be a hard coat layer. The film may also include a thermalbuffer layer partially covering the stabilizing layer, the thermalbuffer layer being patterned to have a variable thickness to define oneor more writable zones and one or more non-writable zones of the film.The thermal buffer layer may have a physical thickness in the one ormore non-writable zones, the physical thickness being at least 5microns. The thermal buffer layer may be substantially absent from,having a zero thickness in, the one or more writable zones.

Related methods, systems, and articles are also discussed.

These and other aspects of the present application will be apparent fromthe detailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in connection with theaccompanying drawings, in which:

FIG. 1 is a schematic side or sectional view of a system in which amultilayer optical film is thermally patterned with a heating element;

FIG. 2 is a schematic perspective view of a system in which a multilayeroptical film is thermally patterned with a plurality of individuallycontrollable heating elements;

FIG. 3 is a schematic side or sectional view of a portion of anexemplary multilayer optical film, the optical film having microlayersin a stack of microlayers arranged into optical repeat units (ORUs), thefilm also having a stabilizing layer and an additional layer near anouter exposed surface;

FIG. 4A is a schematic side or sectional view of a multilayer opticalfilm in combination with a heating element, the heating elementdelivering heat to the film by thermal conduction through the outerexposed surface, and FIG. 4B is a schematic side or sectional view ofthe multilayer optical film after it has been patterned by the deliveryof the heat;

FIG. 5 is a schematic front or plan view of a patterned multilayeroptical film; FIG. 5A is a magnified view of a portion of FIG. 5;

FIG. 6 is an Atomic Force Microscopy (AFM) image of an unaltered regionof a multilayer optical film, the film having a graded layer thicknessprofile, and FIG. 6A is an enlarged AFM image of a portion of themultilayer optical film near one side of the film, and FIG. 6B is anenlarged AFM image of a portion of the multilayer optical film near theopposite side of the film;

FIG. 7 is an AFM image of a first altered region of the multilayeroptical film of FIG. 6, the first altered region being produced bydelivering heat through an outer exposed surface near the thickermicrolayer side of the packet;

FIG. 7A is an enlarged AFM image of a portion of the multilayer opticalfilm of FIG. 7;

FIG. 7B is a graph of the measured transmission of an unaltered regionand of an altered region similar to that of FIGS. 7 and 7A;

FIG. 8 is an AFM image of a second altered region of the multilayeroptical film of FIG. 6, the second altered region being produced bydelivering heat through an outer exposed surface near the thinnermicrolayer side of the packet;

FIG. 8A is an enlarged AFM image of a portion of the multilayer opticalfilm of FIG. 8;

FIG. 8B is a graph of the measured transmission of an unaltered regionand of an altered region similar to that of FIGS. 8 and 8A;

FIG. 9 is a graph that shows the measured spectral transmission ofmultilayer optical film samples that were made and to which heat wasdelivered, the film samples having different thicknesses of stabilizinglayers;

FIG. 10 is a graph of a reflective characteristic (the wavelength of theleft band edge of the reflection band) of the multilayer optical filmsamples of FIG. 9;

FIG. 11 is a schematic side or sectional view of the multilayer opticalfilm after it has been patterned by the delivery of the heat through itsouter exposed surface, the figure illustrating schematically how layerdistortion of the microlayers can occur with certain stabilizing layers;

FIG. 12 is a graph that plots both the wavelength of the measured leftband edge and the measured optical haze of multilayer optical films thatwere made and to which heat was delivered, the film samples havingdifferent thicknesses of stabilizing layers;

FIGS. 13A through 13C show schematic side or sectional views of amultilayer optical film before thermal marking (13A), after thermalmarking (13B), and after application of a thermal buffer layer (13C);and

FIGS. 14A through 14D show schematic side or sectional views of amultilayer optical film before thermal marking (14A), after a firstthermal marking (14B), after application of a patterned thermal bufferlayer (14C), and after a second thermal marking (14D); and

FIG. 15 is a schematic front or plan view of a multilayer optical filmshowing patterning associated with thermal marking and patterningassociated with a patterned thermal buffer layer.

The schematic drawings presented herein are not necessarily to scale;however, graphs are assumed to have accurate scales unless otherwiseindicated. Like numbers used in the figures refer to like elements.

DETAILED DESCRIPTION

As mentioned above, we disclose here inter alia multilayer optical filmshaving at least some birefringent microlayers in a stack of microlayers,the films being designed to be pattern-wise marked using thermalconduction from a heated body that contacts an outer surface of thefilm. The contact between the heated body and the film is preferably asliding contact characterized by a minimal force that allows for boththe sliding contact and efficient thermal contact between the heatedbody and the film. Heat delivered to the film by thermal conductionrelaxes or eliminates the birefringence of some of the birefringentmicrolayers to produce a reflective characteristic that is modified oraltered relative to an original or unaltered reflective characteristic,such reflective characteristics being associated exclusively orpredominantly with the constructive or destructive interference of lightreflected from interfaces within the (altered or unaltered) stack ofmicrolayers. An exemplary system for thermally marking such a film isshown schematically in FIG. 1.

In the figure, a system 110 includes a suitably designed multilayeroptical film 130, which may be in a continuous roll form as shown, or inindividual sheet or piece form if desired. The film 130 includes a stackor packet of microlayers (not shown in FIG. 1), at least some of whichare birefringent. The optical film 130 may also be laminated to adocument or other substrate. The film passes through a marking station120, where it is thermally marked in a desired pattern of alteredregions. At the marking station 120, a heating element 122 of a heatingassembly 124 contacts an outer exposed surface 130 a of the film 130 inorder to deliver heat from the heating element 122 to the film 130 viathermal conduction through such surface 130 a. The heating element canbe of any conventional design, e.g., it may be a heating element from aconventional thermal resistive printer. One or more platens 126 or thelike can be used to feed the film 130 to the marking station at adesired relative film speed, as indicated by arrow 127. The platten(s)126 may rotate as indicated, and may also be used to ensure adequatecontact, e.g. a continuous sliding contact, is maintained between theheating element and the film. The physical contact may be characterizedby a lineal force of the heating element 122 against the platen 126 (andvice versa) of 5 Newtons/cm or less, or 3 Newtons/cm or less. Bymaintaining a continuous sliding contact and modulating the heatingelement 122, e.g., turning it on and off with a suitable controller (notshown) coupled to the heating element 122, one or more altered regions132 are created in the film, which altered regions 132 are separatedfrom each other by unaltered regions 134. The design and control of theheating element 122, e.g., its size, temperature, and power dissipation(which may be a binary “on” and “off” or adjustable over a continuousrange), the design of the film 130, and processing parameters such asthe speed of the film, are selected in such a way that the heatdelivered by the heating element to the film 130 by thermal conductionis sufficient to reduce the birefringence of at least some of thebirefringent microlayers in the packet of microlayers in the localizedregion of the film (i.e. the region of the film immediately adjacent theheating element 122) to change the reflective characteristic of themicrolayer packet. Typically, the heat delivered by the heating elementis preferably not sufficient to completely melt or destroy the layerstructure of at least most of the affected microlayers within themicrolayer packet.

The combination of altered regions 132 and unaltered regions 134 causethe film 130, which originally (before passing through the markingstation 120) may not have been patterned, to be marked with a pattern,the patterned or marked film being indicated by the reference numeral130′. The altered regions 132 may have reflective and/or transmissivecharacteristics that differ sufficiently from those of the unalteredregions 134 to make the pattern of altered and unaltered regions visibleto an ordinary user or observer, e.g., without the need for anyauxiliary instrumentation or detection equipment. Alternatively, thereflective and/or transmissive characteristics of the altered andunaltered regions may differ from each other in a way that is covert ornot discernible, e.g. at least not easily discernible, to the ordinaryuser or observer, but that may be readily detected using suitableinstrumentation or detection equipment such as one or more opticalwavelength filters, optical polarizers, and/or spectroscopicinstruments.

Another system for thermally marking a multilayer optical film is shownschematically in FIG. 2. In this figure, the heating assembly is shownto be in the form of an extended bar with multiple heating elements.However, some features of FIG. 1 are not shown in FIG. 2, even thoughthey may be present. The system 210 of FIG. 2 may thus be the same as,or similar to, the system 110 of FIG. 1. The system 210 of FIG. 2includes a suitably designed multilayer optical film 230. The film 230includes a stack or packet 240 of microlayers 242, at least some ofwhich are birefringent. A typical packet 240 may include tens orhundreds of microlayers 242, the microlayers typically comprisingdifferent thermoplastic materials that can be collectively coextrudedand stretched or otherwise oriented to achieve the desired refractiveindex relationships, including appropriate refractive index differencesalong the x-, y-, and z-directions between adjacent microlayers 242. Inthis regard, the film 230, the packet 240, and the individualmicrolayers 242 are assumed to (at least locally in a region ofinterest) lie in or parallel to an x-y plane of a Cartesian x-y-zcoordinate system, as shown. Further design details of the microlayerpacket of the multilayer optical film are discussed below.

The film 230 has an outer exposed surface 230 a. In the illustratedembodiment, the surface 230 a is also the outer surface of a stabilizinglayer 245 which attaches to and covers the packet 240. To distinguishfrom the optically thin microlayers of the packet 240, the stabilizinglayer 245 has a physical thickness that is substantially greater thanthat of any of the microlayers 242 in the packet 240. For example, ifthe thickest microlayer 242 in the packet 240 has a physical thicknessoft, the stabilizing layer 245 has a physical thickness of at least 2 t.The stabilizing layer 245 typically is or comprises a thermosetmaterial, which may be coated onto the packet 240 after coextrusion andorientation of an initial multilayer optical film, i.e., onto aninitially uncoated multilayer optical film that is a precursor to thecoated film 230 shown in FIG. 2. Further details of suitable stabilizinglayers are provided below. In general, the stabilizing layer providessufficient heat conduction to allow heat from a heating element inphysical contact with the outer surface 230 a to reduce thebirefringence of the birefringent microlayers disposed near the outersurface 230 a in the altered region being heated, while providingsufficient mechanical support to avoid substantial layer distortion ofthe microlayers near the outer surface 230 a in such altered region, asexplained further below.

The film 230 also has a second outer exposed surface 230 b, which is ona side of the film opposite to that of the outer surface 230 a. Thisother side of the film may have a number of different possibleconfigurations: it may terminate at the end of the packet 240 ofmicrolayers; it may terminate at an optically thick light transmissivepolymer layer that attaches to and covers the packet 240; or it may bebonded to a non-polymer substrate such as a paper or document—in whichcase the surface would be an “outer” surface of the film but would notbe “exposed”, due to its attachment to the paper or document. In theillustrated embodiment, the film 230 includes an optically thick lighttransmissive layer, shown as layer 248. The layer 248 may be or compriseanother stabilizing layer the same as or similar to layer 245. As such,the layer 248 may comprise a thermoset material, and may have a suitablecomposition and thickness to provide sufficient heat conduction to allowheat from a heating element (which may be the same as or similar to theheating elements 222) in physical contact with the outer surface 230 bto reduce the birefringence of the birefringent microlayers disposednear the outer surface 230 b in the altered region being heated, whileproviding sufficient mechanical support to avoid substantial layerdistortion of the microlayers near the outer surface 230 a in suchaltered region, as explained further below. Note in this regard thataltered regions of such a film embodiment can be created by heatedmarking at the outer surface 230 a (via heating assembly 224) and canalso be created by heated marking at the opposite outer surface 230 b(via a heating assembly which may be the same as or similar to assembly224 but positioned at the surface 230 b). By including the layer 248 anddesigning it to be the same as or similar to the stabilizing layer 245on the other side of the film 230, the film 230 can possess a roughlybalanced or symmetrical construction—if we ignore any layer thicknessgradient along the z-axis that may be present in the packet 240, thefilm 230 may be considered to have a substantial mirror symmetryrelative to a reference plane parallel to the x-y plane but passingthrough the center of the packet 240 and through the center of the film230. This construction symmetry can have the advantage of substantiallyreduce curling or warping of the film 230. In alternative embodimentsthat have similar construction symmetries, the layer 248 may be anoptically thick skin layer, e.g. made of a thermoplastic material thatis coextruded with the microlayers.

The heating assembly 224 is in the form of an extended bar which iselongated along a particular in-plane direction, such as the x-axis. Theassembly 224 includes a plurality of heating elements 222. The heatingelements 222 are spaced along an axis that is at least partiallytransverse to the direction of motion of the film relative to theheating assembly 224. The heating elements 222 may thus be spaced, forexample, along the x-axis, while the relative motion of the film 230 isalong the y-axis, indicated by arrow 227. The heating elements 222 makephysical contact with the outer exposed surface 230 a of the film 230 ata marking station 220 to permit heat to be delivered to the packet 240by thermal conduction across the surface 230 a and through thestabilizing layer 245. In a typical embodiment, a relatively lightlineal force, e.g. no more than 5 Newtons/cm, or no more than 3Newtons/cm, is applied between any given heating element 222 and thefilm 230, so as to permit a sliding motion between the heating assembly224 and the film 230. The heating elements 222 may be individuallyaddressable so that the heating elements 222 in combination can providea desired heat or power dissipation profile as a function of positionthat can also be made to change rapidly with time as the film 230 passesthe heating assembly 224. In this way, a pattern that has spatialvariability in both the x- and y-directions can be made with a singlepass of the film 230 across the heating assembly 224. The heatingassembly 224 may be or comprise any suitable heating assembly, e.g., aheating assembly from a conventional resistive thermal printer, e.g.,110XIIIIPLUS Industrial Printer made by Zebra Technologies, Linconshire,Ill. Relative motion between the film 230 and the heating assembly 224can also be provided in whole or in part by mounting the heatingassembly 224 on a moveable mechanism such as a single-axis scanner or anx-y scanner.

In FIG. 3, a portion of a multilayer optical film 330 is shown inschematic side or sectional view to reveal the structure of the filmincluding some of its interior layers. In this particular embodiment,the film 330 includes not only a packet 340 of microlayers 342 and astabilizing layer 345 attached to and covering the packet 340, but alsoa lubricant layer 349 which is attached to and covers the stabilizinglayer 345 (as well as the packet 340). In this embodiment, the lubricantlayer is the outermost layer on one side of the film 330, and has amajor surface coinciding with the outer exposed surface 330 a of thefilm 330. The lubricant layer 349 may be composed of a non-polymerlubricant material such as wax. The lubricant layer is generallycomprised of a heat resistant binder and, optionally, one or morelubricants, as well as one or more abrasive particles that may beprovided to clean the print head of any accumulated thermal degradationproducts and extend the print head life. The binders may be crosslinkedsilicones, polyurethanes, polyvinylalcohols, polyacrylates, polyesters,epoxies, or the like. The lubricant is added to reduce the coefficientof friction between the media and the thermal print head. Lubricants maybe comprised of metal salts of high fatty acids such as zinc stearate,calcium stearate, zinc stearylphosophate, waxes such as paraffin,polyethylene, canauba, candelilla, microcrystalline phosphate esters andthe like, and silicone olidgomers, fluoro-additives, surfactants and thelike. Reference in this regard is made to patent applicationpublications US 2011/0251060 (Harrison et al.) for DT-like waxes and US2005/0162493 (Gross) for fluoro additives and surfactants, and U.S. Pat.No. 7,829,162 (Eskra et al.) for TT-like binders. The lubricant layercan facilitate sliding contact between the heating element(s) and thefilm 330, e.g. by reducing friction therebetween. The stabilizing layer345 may be the same as or similar to stabilizing layer 245, discussedabove.

The multilayer optical film has reflective and transmissive propertiesor characteristics which are predominantly due to constructive anddestructive interference of light reflected from layer interfacesbetween the microlayers in the packet, see e.g. microlayers 342 inpacket 340. Typically, but not necessarily, the multilayer optical filmis at least partially light transmissive. In general, transmission (T)plus reflection (R) plus absorption (A)=100%, or T+R+A=100%. In someembodiments the film may be composed entirely of materials that have lowabsorption over at least a portion of the wavelength spectrum. Thus, inmany cases the multilayer optical film may have an absorption that issmall or negligible over at least a limited portion of the wavelengthspectrum, such as the visible spectrum, in which case the reflection andtransmission over that limited range take on a complementaryrelationship because T+R=100%−A, and since A is small,T+R≈100%.

In FIG. 3 and many of the other figures, the multilayer optical film isshown in relation to a local x-y-z Cartesian coordinate system, wherethe film extends parallel to the x- and y-axes, and the z-axis isperpendicular to the film and its constituent layers and parallel to athickness axis of the film. This is not intended to be limiting, sinceeven if the film is curved or otherwise shaped to deviate from a plane,arbitrarily small portions or regions of the film can be associated witha local Cartesian coordinate system as shown. For simplicity we willassume that the portion of the film 330 shown in FIG. 3 is in itsunaltered state, e.g., after coextrusion, orientation, and coating, butbefore being brought into contact with any heating elements such asthose shown in FIGS. 1 and 2. The individual layers 342, 345, 349 of thefilm are assumed to extend continuously so as to be coextensive witheach other and with the film 330, whatever its physical size may be.

As stated earlier, the microlayers of the multilayer optical film aresufficiently thin so that light reflected at a plurality of theinterfaces undergoes constructive or destructive interference to givethe multilayer optical film the desired reflective or transmissiveproperties. For multilayer optical films designed to reflect light atultraviolet, visible, or near-infrared wavelengths, each microlayergenerally has an optical thickness (a physical thickness multiplied byrefractive index) of less than about 1 μm. However, thicker layers canalso be included, such as skin layers at the outer surfaces of themultilayer optical film, or protective boundary layers (PBLs) disposedwithin the multilayer optical film to separate coherent groupings (knownas “stacks” or “packets”) of microlayers. In FIG. 3, the microlayers arelabeled “A” or “B”, the “A” layers being composed of one material andthe “B” layers being composed of a different material, these layersbeing stacked in an alternating arrangement to form optical repeat unitsor unit cells which are labeled “ORU”. Typically, a multilayer opticalfilm composed entirely of polymeric materials would include many morethan 3 or 4 optical repeat units if high reflectivities are desired.Note that all of the “A” and “B” microlayers shown in FIG. 3 areinterior layers of film 330. If desired, two or more separate multilayeroptical films can be laminated together, e.g. with one or more adhesivelayers, or using pressure, heat, or other methods to form a laminate orcomposite film.

In some cases, the microlayers can have thicknesses and refractive indexvalues corresponding to a ¼-wave stack, i.e., arranged in optical repeatunits each having two adjacent microlayers of equal optical thickness(f-ratio=50%, the f-ratio being the ratio of the optical thickness of aconstituent layer “A” to the optical thickness of the complete opticalrepeat unit), such optical repeat unit being effective to reflect byconstructive interference light whose wavelength λ is twice the overalloptical thickness of the optical repeat unit, where the “opticalthickness” of a body refers to its physical thickness multiplied by itsrefractive index. In other cases, the optical thickness of themicrolayers in an optical repeat unit may be different from each other,whereby the f-ratio is greater than or less than 50%. Each opticalrepeat unit ORU shown in FIG. 3 has an optical thickness OT equal to thesum of the optical thicknesses of its constituent “A” and “B” layer, andeach optical repeat unit reflects light whose wavelength λ is twice itsoverall optical thickness. The reflectivity provided by microlayerstacks or packets used in multilayer optical films in general, and bythe patterned multilayer optical films discussed herein in particular,is typically substantially specular in nature, rather than diffuse, as aresult of the generally smooth well-defined interfaces betweenmicrolayers, and the low haze materials that are used in a typicalconstruction. In some cases, however, the finished article may betailored to incorporate any desired degree of scattering, e.g., using adiffuse material in skin layer(s) and/or PBL layer(s), and/or using oneor more surface diffusive structures or textured surfaces, for example.

In some embodiments, the optical thicknesses of the optical repeat unitsin a packet of microlayers may all be equal to each other, to provide anarrow reflection band of high reflectivity centered at a wavelengthequal to twice the optical thickness of each optical repeat unit. Inother embodiments, the optical thicknesses of the optical repeat unitsmay differ according to a thickness gradient along the z-axis orthickness direction of the film, whereby the optical thickness of theoptical repeat units increases, decreases, or follows some otherfunctional relationship as one progresses from one side of the stack(e.g. the top) to the other side of the stack (e.g. the bottom). Suchthickness gradients can be used to provide a widened reflection band toprovide substantially spectrally flat transmission and reflection oflight over the extended wavelength band of interest, and also over allangles of interest. Thickness gradients tailored to sharpen the bandedges at the wavelength transition between high reflection and hightransmission can also be used, as discussed in U.S. Pat. No. 6,157,490(Wheatley et al.) “Optical Film With Sharpened Bandedge”. For polymericmultilayer optical films, reflection bands can be designed to havesharpened band edges as well as “flat top” reflection bands, in whichthe reflection properties are essentially constant across the wavelengthrange of application. Other layer arrangements, such as multilayeroptical films having 2-microlayer optical repeat units whose f-ratio isdifferent from 50%, or films whose optical repeat units include morethan two microlayers, are also contemplated. These alternative opticalrepeat unit designs can be configured to reduce or to excite certainhigher-order reflections, which may be useful if the desired reflectionband resides in or extends to near infrared wavelengths. See, e.g., U.S.Pat. No. 5,103,337 (Schrenk et al.) “Infrared Reflective OpticalInterference Film”, U.S. Pat. No. 5,360,659 (Arends et al.) “TwoComponent Infrared Reflecting Film”, U.S. Pat. No. 6,207,260 (Wheatleyet al.) “Multicomponent Optical Body”, and U.S. Pat. No. 7,019,905(Weber) “Multi-layer Reflector With Suppression of High OrderReflections”.

The thickness gradient and optical repeat unit design may thus betailored as desired to provide the disclosed multilayer optical films,whether in an unaltered (untreated) or altered (treated) region thereof,and whether for light of one polarization state or for unpolarizedlight, with a substantial reflectivity in a limited spectral band. Forexample, the substantial reflectivity may be at least 50%, or at least60, 70, 80, or 90% or more, over only substantially one spectral band,the band being disposed in the visible or in any other desired portionof the spectrum. The band may have a bandwidth of less than 200, or 150,or 100, or 50 nm or less, for example, which may be measured as afull-width at half-maximum (FWHM) reflectivity. The band may beassociated with zero-order reflection, or with a desired higher orderreflection if the optical repeat unit is suitably designed.

As mentioned above, adjacent microlayers of the multilayer optical filmhave different refractive indices so that some light is reflected atinterfaces between adjacent layers. We refer to the refractive indicesof one of the microlayers (e.g. the “A” layers in FIG. 3) for lightpolarized along principal x-, y-, and z-axes as n1x, n1y, and n1z,respectively. We refer to the refractive indices of the adjacentmicrolayer (e.g. the “B” layers in FIG. 3) along the same axes as n2x,n2y, n2z, respectively. The x-, y-, and z-axes may, for example,correspond to the principal directions of the dielectric tensor of thematerial. Typically, and for discussion purposes, the principledirections of the different materials are coincident, but this need notbe the case in general. We refer to the differences in refractive indexbetween these layers as Δnx (=n1x−n2x) along the x-direction, Δny(=n1y−n2y) along the y-direction, and Δnz (=n1z−n2z) along thez-direction. The nature of these refractive index differences, incombination with the number of microlayers in the film (or in a givenstack of the film) and their thickness distribution, controls thereflective and transmissive characteristics of the film (or of the givenstack of the film) in a given region or zone. For example, if adjacentmicrolayers have a large refractive index mismatch along one in-planedirection (Δnx large) and a small refractive index mismatch along theorthogonal in-plane direction (Δny≈0), the film or packet may behave asa reflective polarizer for normally incident light. In this regard, areflective polarizer may be considered for purposes of this applicationto be an optical body that strongly reflects normally incident lightthat is polarized along one in-plane axis (referred to as the “blockaxis”) if the wavelength is within the reflection band of the packet,and strongly transmits such light that is polarized along an orthogonalin-plane axis (referred to as the “pass axis”). “Strongly reflects” and“strongly transmits” may have different meanings depending on theintended application or field of use, but in many cases a reflectivepolarizer will have at least 70, 80, or 90% reflectivity for the blockaxis, and at least 70, 80, or 90% transmission for the pass axis.

For purposes of the present application, a material is considered to be“birefringent” if the material has an anisotropic dielectric tensor overa wavelength range of interest, e.g., a selected wavelength or band inthe UV, visible, and/or infrared portions of the spectrum. Stateddifferently, a material is considered to be “birefringent” if theprincipal refractive indices of the material (e.g., n1x, n1y, n1z) arenot all the same.

In another example, adjacent microlayers may have a large refractiveindex mismatch along both in-plane axes (Δnx large and Δny large), inwhich case the film or packet may behave as an on-axis mirror. In thisregard, a mirror or mirror-like film may be considered for purposes ofthis application to be an optical body that strongly reflects normallyincident light of any polarization if the wavelength is within thereflection band of the packet. Again, “strongly reflecting” may havedifferent meanings depending on the intended application or field ofuse, but in many cases a mirror will have at least 70, 80, or 90%reflectivity for normally incident light of any polarization at thewavelength of interest.

In variations of the foregoing embodiments, the adjacent microlayers mayexhibit a refractive index match or mismatch along the z-axis (Δnz≈0 orΔnz large), and the mismatch may be of the same or opposite polarity orsign as the in-plane refractive index mismatch(es). Such tailoring ofΔnz plays a key role in whether the reflectivity of the p-polarizedcomponent of obliquely incident light increases, decreases, or remainsthe same with increasing incidence angle. In yet another example,adjacent microlayers may have a substantial refractive index match alongboth in-plane axes (Δnx≈Δny≈0) but a refractive index mismatch along thez-axis (Δnz large), in which case the film or packet may behave as aso-called “p-polarizer”, strongly transmitting normally incident lightof any polarization, but increasingly reflecting p-polarized light ofincreasing incidence angle if the wavelength is within the reflectionband of the packet.

In still another example, adjacent microlayers may both be birefringentand may have refractive indices that match or substantially match eachother along all three principal axes, i.e., Δnx≈Δny≈Δnz≈0. By matchingthe refractive indices in this way, the film provides little or noreflectivity, and high transmission, such that the film has theappearance of a window or single monolithic layer of transparentmaterial. The “reflective characteristic” for such a multilayer opticalfilm is thus a window characteristic, with little or no reflectivity.This characteristic can however be modified, e.g. in an altered regionby the delivery of an appropriate amount of heat, to a reflectivecharacteristic with greater reflectivity, such as a reflective polarizeror mirror characteristic. This is accomplished by adjusting the heat sothat it is high enough to reduce the birefringence of one of the twotypes of microlayers, e.g. the A microlayers, but low enough so that thebirefringence of the other microlayer type, e.g. the B microlayers, ismaintained, or reduced by a lesser amount. The A and B microlayers insuch embodiments are typically selected to have substantially differentmaterial properties, such as different glass transition temperatures.

In view of the large number of permutations of possible refractive indexdifferences along the different axes, the total number of layers andtheir thickness distribution(s), and the number and type of microlayerpackets included in the multilayer optical film, the variety of possiblemultilayer optical films and packets thereof is vast. We refer tomultilayer optical films disclosed in any of the patent documents citedherein (whether or not patented, and whether published by the U.S.Patent Office or by any another country or patent authority), as well asthe following documents, all of which are incorporated herein byreference: U.S. Pat. No. 5,486,949 (Schrenk et al.) “BirefringentInterference Polarizer”; U.S. Pat. No. 5,882,774 (Jonza et al.) “OpticalFilm”; U.S. Pat. No. 6,045,894 (Jonza et al.) “Clear to Colored SecurityFilm”; U.S. Pat. No. 6,179,949 (Merrill et al.) “Optical Film andProcess for Manufacture Thereof”; U.S. Pat. No. 6,531,230 (Weber et al.)“Color Shifting Film”; U.S. Pat. No. 6,939,499 (Merrill et al.)“Processes and Apparatus for Making Transversely Drawn Films withSubstantially Uniaxial Character”; U.S. Pat. No. 7,256,936 (Hebrink etal.) “Optical Polarizing Films with Designed Color Shifts”; U.S. Pat.No. 7,316,558 (Merrill et al.) “Devices for Stretching Polymer Films”;PCT Publication WO 2008/144136 A1 (Nevitt et al.) “Lamp-Hiding Assemblyfor a Direct Lit Backlight”; PCT Publication WO 2008/144656 A2 (Weber etal.) “Backlight and Display System Using Same”. Furthermore, themultilayer optical film may have an initial reflective characteristicbefore marking (or in an unaltered region) selected from a mirror,polarizer, or window characteristic, and after marking in an alteredregion the film may have a modified reflective characteristic that isalso selected from a mirror, polarizer, or window characteristic, in anypermutation. The reflective characteristic may for example change froman initial mirror characteristic to a modified mirror characteristic, orfrom an initial mirror characteristic to a modified polarizercharacteristic or a modified window characteristic. Further discussionin this regard can be found in international patent applicationpublication WO 2010/075373 (Merrill et al.).

At least some of the microlayers in the packet to be marked or patternedare birefringent before the selective heat treatment, and preferablyalso are birefringent in at least one region or zone (an “unalteredregion”) of the finished film after heat treatment. Thus, a first layerin the optical repeat units of a particular layer packet may bebirefringent (i.e., n1x≠n1y, or n1x≠n1z, or n1y≠n1z), or a second layerin the optical repeat units of such packet may be birefringent (i.e.,n2x≠n2y, or n2x≠n2z, or n2y≠n2z), or both the first and second layersmay be birefringent. Moreover, the birefringence of one or more suchlayers is diminished in at least one region (an “altered region”)relative to a neighboring region. In some cases, the birefringence ofthese layers may be diminished to zero, such that they are opticallyisotropic (i.e., n1x=n1=n1z, or n2x=n2y=n2z) in an altered region butbirefringent in a neighboring unaltered region. In cases where bothlayers are initially birefringent, depending on materials selection andprocessing conditions, they can be locally heated in such a way that thebirefringence of only one of the layers is substantially diminished, asdiscussed above, or the birefringence of both layers may be diminished.

Exemplary multilayer optical films and microlayer packets thereof arecomposed of polymer materials and may be fabricated using coextruding,casting, and orienting processes. Reference is made to U.S. Pat. No.5,882,774 (Jonza et al.) “Optical Film”, U.S. Pat. No. 6,179,949(Merrill et al.) “Optical Film and Process for Manufacture Thereof”, andU.S. Pat. No. 6,783,349 (Neavin et al.) “Apparatus for Making MultilayerOptical Films”. The multilayer optical film may be formed by coextrusionof the polymers as described in any of the aforementioned references.The polymers of the various layers are preferably chosen to have similarrheological properties, e.g., melt viscosities, so that they can beco-extruded without significant flow disturbances. Extrusion conditionsare chosen to adequately feed, melt, mix, and pump the respectivepolymers as feed streams or melt streams in a continuous and stablemanner. Temperatures used to form and maintain each of the melt streamsmay be chosen to be within a range that avoids freezing,crystallization, or unduly high pressure drops at the low end of thetemperature range, and that avoids material degradation at the high endof the range.

In brief summary, the fabrication method may comprise: (a) providing atleast a first and a second stream of resin corresponding to the firstand second polymers to be used in the finished film; (b) dividing thefirst and the second streams into a plurality of layers using a suitablefeedblock, such as one that comprises: (i) a gradient plate comprisingfirst and second flow channels, where the first channel has across-sectional area that changes from a first position to a secondposition along the flow channel, (ii) a feeder tube plate having a firstplurality of conduits in fluid communication with the first flow channeland a second plurality of conduits in fluid communication with thesecond flow channel, each conduit feeding its own respective slot die,each conduit having a first end and a second end, the first end of theconduits being in fluid communication with the flow channels, and thesecond end of the conduits being in fluid communication with the slotdie, and (iii) optionally, an axial rod heater located proximal to saidconduits; (c) passing the composite stream through an extrusion die toform a multilayer web in which each layer is generally parallel to themajor surface of adjacent layers; and (d) casting the multilayer webonto a chill roll, sometimes referred to as a casting wheel or castingdrum, to form a cast multilayer film. This cast film may have the samenumber of layers as the finished film, but the layers of the cast filmare typically much thicker than those of the finished film. Furthermore,the layers of the cast film are typically all isotropic.

Many alternative methods of fabricating the cast multilayer web can alsobe used. One such alternative method that also utilizes polymercoextrusion is described in U.S. Pat. No. 5,389,324 (Lewis et al.).

After cooling, the multilayer web can be drawn or stretched to producethe near-finished multilayer optical film, details of which can be foundin the references cited above. The drawing or stretching accomplishestwo goals: it thins the layers to their desired final thicknesses, andit orients the layers such that at least some of the layers becomebirefringent. The orientation or stretching can be accomplished alongthe cross-web direction (e.g. via a tenter), along the down-webdirection (e.g. via a length orienter), or any combination thereof,whether simultaneously or sequentially. If stretched along only onedirection, the stretch can be “unconstrained” (wherein the film isallowed to dimensionally relax in the in-plane direction perpendicularto the stretch direction) or “constrained” (wherein the film isconstrained and thus not allowed to dimensionally relax in the in-planedirection perpendicular to the stretch direction). If stretched alongboth in-plane directions, the stretch can be symmetric, i.e., equalalong the orthogonal in-plane directions, or asymmetric. Alternatively,the film may be stretched in a batch process. In any case, subsequent orconcurrent draw reduction, stress or strain equilibration, heat setting,and other processing operations can also be applied to the film.

Additional layers and coatings, selected for their optical, mechanical,and/or chemical properties, can also then be applied to the multilayeroptical film. Of particular importance is a stabilizing layer asmentioned above and described in further detail below. The stabilizinglayer typically comprises a thermoset material and is thin enough toprovide sufficient heat conduction to allow heat from a heating elementto reduce the birefringence of the birefringent microlayers disposednear the outer exposed surface of the film, but is also thick enough toprovide sufficient mechanical support to avoid substantial layerdistortion of such microlayers, which layer distortion may be manifestedas optical haze. The stabilizing layer may be used to ensure the opticalhaze in the altered region(s) does not exceed 20%, or does not exceed10%. Some thermoset materials that may be used for the stabilizing layerinclude epoxide resins, (meth)acrylate resins, polyester resins, urearesins, melamine resins, or polyurethane resins, polyimide resins, andcyanate esters. These materials may comprise UV-curable functionalgroups, typically UV-curable resins comprise olefin-functional monomersand olefin-functional oligomers and polymers. These materials may alsocomprise sol-based functionalized inorganic particles for improvedmechanical properties. The stabilizing layer may also serve as a hardcoat for the film. That is, the hard coat may impart improved durabilityto the film. The stabilizing layer or hard coat may provide a protectivelayer for the film; providing resistance to abrasion, weathering, and/orchemical attack.

Another possible additional layer of interest is a lubricant layer, asdiscussed above.

In the marking process, heat is delivered to the film from a heatingelement in physical contact with an outer exposed surface of the film.Such a procedure is shown schematically in FIG. 4A. In that figure, amultilayer optical film 430 having an outer exposed surface 430 a is inphysical contact with a heating element 422 of a heating assembly 424.The contact may be a sliding contact with minimal force or pressure, andthe film 430 may be in motion e.g. along the positive or negativex-direction relative to the heating assembly 424, and the heatingelement 422 may be modulated or controlled from an OFF state (e.g., notenergized, or cool) to a fully ON state (e.g., at a maximum power leveland maximum temperature), and in many cases to intermediate statesbetween OFF and fully ON, e.g., at discrete power levels correspondingto a pixel bit depth for gray scale patterning. Heating assembly thermalmaintenance algorithms may also be used, where the power applied to agiven heating element at any given time depends on the pixel that isbeing printing directly beneath it at that time, as well as one or moreneighboring pixels that were printed before the given time, and one ormore neighboring pixels that are to be printed after the given time. InFIG. 4A, the heating element 422 is assumed to be ON, whether fully orat an intermediate power level. Heat flows into the film 430 via thermalconduction across the outer exposed surface 430 and through any layer orlayers near that surface, e.g., through a stabilizing layer 445 of thefilm 430. The multilayer optical film 430 may in this regard be the sameas or similar to other multilayer optical films discussed herein. Thus,the film 430 includes at least one packet 440 of microlayers 442, thepacket 440 typically being made by coextrusion of thermoplasticmaterials which form the microlayers. At least some of the microlayers442 are birefringent. For example, in an alternating AB configurationsuch as that of FIG. 3, every other microlayer, such as all of the Amicrolayers, or all of the B microlayers, may be birefringent. Thelayer-to-layer differences in refractive index Δnx, Δny, and Δnz, alongwith other factors such as the total number of microlayers and theirrespective thicknesses, provide the film 430 with an initial reflectivecharacteristic, e.g., a mirror characteristic or a polarizercharacteristic over a certain spectral band or bands of interest.

Heat from the heating element 422 is delivered in such a way thatportions of some of the microlayers 442 nearest the heating element 422are heated to a point that is close enough to a melting or softeningpoint (e.g. a glass transition temperature) of the respective materials,and of appropriate duration or dwell time, that any birefringencepossessed by such layers is reduced. This reduction in birefringence maybe partial or complete. The amount of heat delivered, which is afunction of many parameters such as the physical size, temperature, andpower dissipation of the heating element, the contact area between theheating element and the outer exposed surface, and the duration of theheating (e.g. as may be controlled by the speed of the film relative tothe heating assembly), is typically enough to provide such materialrelaxation for microlayers that are closest to the heating element, buttypically not so much that the layer structure of most of the affectedmicrolayers is destroyed. Rather, the layer structure of most of theaffected microlayers is typically preserved, to the extent that themicrolayers can still be distinguished from each other, e.g. due to lowcontrast levels between the materials comprising the individualmicrolayers in the affected zones, using suitable analyticalinstrumentation and techniques, e.g., atomic force microscopy asdescribed below.

The material relaxation, and reduction in birefringence, is usuallylimited to a portion of the film nearest the heating element as depictedgenerally by a critically heated volume 432 in FIG. 4A. Inside thecritically heated volume 432, microlayers 442 that were initiallybirefringent become less birefringent, including in some casesisotropic. Outside the critically heated volume 432, microlayers 442that were initially birefringent substantially retain theirbirefringence, and substantially retain their original indices ofrefraction associated with the respective x-, y-, and z-axes. Thus,microlayers 442 that are located deep within the film 430, and/or thatare closer to the outer surface 430 b than to the surface 430 a, mayexperience no significant reduction in birefringence at any (x,y)position. On the other hand, microlayers 442 that are located near theouter surface 430 a through which heat is delivered may experience areduction in birefringence at some (x,y) positions, namely, those (x,y)positions that lie within the critically heated volume 432, whileexperiencing no significant reduction in birefringence at other (x,y)positions, namely, those (x,y) positions that lie outside of the volume432. Reductions in the birefringence of the microlayers may not be thesame throughout the critically heated volume 432, but may be greatestnear the outer exposed surface, i.e., near the source of the heat, andleast, e.g., at or near the limit of detectability, at the boundaries ofthe volume 432 farthest away from the source of heat. Consequently, in amultilayer optical film as patterned herein, a group of firstbirefringent microlayers may have respective refractive indices that aresubstantially unchanged in altered regions relative to unalteredregions, and a group of second birefringent microlayers may haverespective refractive indices that are substantially changed in thealtered regions relative to the unaltered regions, the group of secondmicrolayers being closer than the group of first microlayers to theouter exposed surface 430 a.

The process depicted in FIG. 4A has the effect of marking the film 430by altering the birefringence of material layers in the criticallyheated volume 432. After the heating element 422 is turned OFF orotherwise removed, all portions of the film 430 return to ambienttemperature. The resultant marked multilayer optical film is shown inFIG. 4B, where it is labeled 430′ to distinguish it from the multilayeroptical film 430 in its original condition before the marking process.Otherwise, like reference numerals refer to like elements. Some of themicrolayers in the marked film 430′ are labeled 442′ to indicate that atleast a portion of such microlayers lie within the volume 432 and thushave a reduced birefringence in such volume. For this reason also, thepacket of microlayers is labeled 440′ due to the presence of suchpatterned microlayers.

For an observer or user of the film 430′, the reduced birefringence ofmicrolayers within the volume 432 has the effect of changing thereflective characteristic of the film relative to its originalreflective characteristic. The change in reflective characteristic islocalized, due to the localized nature of the volume 432. Thus, thevolume 432 provides a basis from which we can identify an altered region436 and unaltered regions 434 of the patterned multilayer optical film430′. In the unaltered regions 434, the microlayers of the packet 440′,as evaluated along an axis parallel to the z-axis or thickness axis ofthe film 430′, all have substantially the same respective refractiveindices (and thus substantially the same birefringence) as they did inthe original (pre-marked) multilayer optical film 430. The reflectivecharacteristic of the film 430′ in those unaltered regions 434 istherefore substantially the same as the reflective characteristic of theoriginal film 430. On the other hand, in the altered region 436, asubstantial number of the microlayers of the packet 440′, evaluated inthe same way along the thickness axis of the film, have substantiallydifferent refractive indices and reduced birefringence compared to theirrefractive indices and birefringence in the original film 430. Despitethe fact that some of the microlayers in the altered region 436 (e.g.the microlayers close to the outer surface 430 b) are unchanged relativeto the original film 430, the presence of the modified microlayers 442′,with their reduced birefringence and different refractive indicesrelative to the original film 430, results in a reflectivecharacteristic that is different in the altered region 436 than theoriginal reflective characteristic associated with the unaltered regions434. As mentioned above, the difference in the reflectivecharacteristics (and/or in the transmissive characteristics) may beeasily observable or noticeable to an ordinary user or observer, or theymay be covert or not discernible to the ordinary user or observer, butdetectable using auxiliary instrumentation or detection equipment.

By carrying out the marking process of FIG. 4A as many times asnecessary or desired, the multilayer optical film may be marked with a2-dimensional pattern of arbitrary design. The pattern may be orcomprise any of alphanumeric characters, symbols, lines, dots, grayscaleimages, or any other features or marks. A marked or patterned multilayeroptical film 530 having a particular pattern of altered regions is shownschematically in FIGS. 5 and 5A. In this case, the film 530 hasunaltered regions 534 and altered regions 536, which may be made usingthe approach of FIG. 4A. One of the altered regions 536 in FIG. 5 ismagnified in FIG. 5A, which reveals that this altered region is actuallyan array of smaller altered regions 536 separated by unaltered regions534.

A multilayer optical film was fabricated and thermally marked withseveral patterns including the patterns shown in FIG. 5, and analyzed.In its original (pre-marked) state, the multilayer optical film had thefollowing features:

-   -   it had a single packet of microlayers which were formed using        coextrusion, the microlayers being in an alternating AB        configuration as shown generally in FIG. 3 where each ORU        consisted of one A microlayer and one B microlayer, and for any        given ORU the A and B microlayers were about the same thickness        for an f-ratio of roughly 50%;    -   there were about 153 A microlayers and about 152 B microlayers        for a total of about 305 microlayers in the packet;    -   the B microlayers were composed of a polyester copolymer        comprised of 90 mol % naphthalate and 10 mol % terephthalate on        an esters basis, i.e., a 90/10 mol % first copolymer of PEN and        PET sub-unit (comprising 90 mol % naphthalene dicarboxylate, 10        mol % terephthalate as the carboxylate of Example 1 of U.S. Pat.        No. 6,352,761 (Hebrink et al.), so-called “low melt PEN” or        “LmPEN”;    -   the A microlayers were composed of a copolyester resulting from        the in-situ transesterification of a polymer blend of about 50%        of the low melt PEN and about 50% of so-called PETg, PETg being        a polyester copolymer comprised of about 70 mol % ethylene        glycol and about 30 mol % cyclohexanediol on a diol basis,        available as Eastar™ GN071 (Eastman Chemicals, Kingsport, Tenn.,        USA);    -   the B microlayers were isotropic, with a refractive index of        1.596 at 632.8 nm;    -   the A microlayers were birefringent, with refractive indices nx,        ny, nz of about 1.82, 1.54, and 1.49, respectively, also        measured at 632.8 nm;    -   the microlayers and ORUs were characterized by a layer thickness        gradient along the thickness axis of the film, with thinner ORUs        (and microlayers) on a first side of the packet, ORUs (and        microlayers) of intermediate thickness in the middle of the        packet, and thicker ORUs (and microlayers) on a second side of        the packet;    -   one optically thick PBL contacted and covered the first side,        and another optically thick PBL contacted and covered the second        side, of the microlayer packet, each PBL having a physical        thickness of about 1 micron, the outer surfaces of these PBLs        forming the outer surfaces of the multilayer optical film, and        these PBLs being made of the same LmPEN as the B microlayers        (and having an isotropic refractive index of about 1.596 at        632.8 nm) and being made by coextrusion with the microlayers.        As can be seen by comparing the refractive index of the        isotropic A microlayers with the refractive indices of the        birefringent B microlayers, Δnx is relatively large and Δny≈0.        The multilayer optical film thus functioned as a reflective        polarizer, having a high reflectivity (and low transmission) for        light polarized along the x-axis, referred to also as the block        axis of the polarizer, and a low reflectivity (and high        transmission) for light polarized along the y-axis, referred to        as the pass axis of the polarizer. Due to the thickness gradient        of the ORUs in the microlayer packet, the reflectivity        associated with the block axis was over an extended reflection        band which had a left band edge (a short wavelength band edge)        at about 400 nm and a right band edge (a long wavelength band        edge) at about 1000 nm. In accordance with the above discussion        of optical repeat units and f-ratios, this translates to the A        and B microlayers each having optical thicknesses of about 100        nm (physical thicknesses of roughly 50 nm for the A microlayers        and 60 nm for the B microlayers) at the first side of the        packet, and increasing in thickness along the thickness axis of        the film until at the second side of the packet, the A and B        microlayers each have optical thicknesses of about 250 nm        (physical thicknesses of roughly 140 nm for the A microlayers        and 160 nm for the B microlayers). The (physical) thickness of        each PBL was thus greater than a factor of 2, and even greater        than a factor of 3, 4, or 5, times the (physical) thickness of        the thickest microlayer in the packet of microlayers.

With its broad reflection band encompassing virtually all of the visibleregion (about 400 to 700 nm) and into the near infrared for onepolarization (block state), and its high transmission for the oppositepolarization (pass state), the multilayer optical film, which was areflective polarizer, had the appearance in unpolarized light of asilvery partial reflector.

This multilayer optical film was then marked, treated, or patterned bysubjecting it to heat treatment generally as described above inconnection with FIGS. 1, 2, and 4A. This was accomplished with acommercially available direct/thermal transfer printer, model110XIIIIPLUS Industrial Printer made by Zebra Technologies, Linconshire,Ill. The printer was configured in the direct writing mode to applymaximum thermal power to the sheeting. In operation, a piece or sheet ofthe multilayer optical film was fed into the printer, which has amechanism for moving the sheet in a stepped fashion (stop-and-go, thesheet being stopped under the print head for a given residence time foreach pixel, but an average speed of 4 inches per second) past a heatingassembly (print head). The heating assembly has a multitude ofindividually controllable heating elements, the size and spacing ofwhich is configured to provide a resolution of 300 dots per inch (dpi).The lineal force or pressure between the sheet and the heating elements,as described athttp://global.kyocera.com/prdct/printing-devices/thermal-printheads/tec/platen.html,is 0.3 kgw/cm, where kgw refers to a unit of force equal to the weightof one kilogram at the Earth's surface. Under computer control, theprinter, and in particular its heating assembly, can be made to impartany desired 2-dimensional pattern onto the sheet by appropriatelyenergizing and modulating individual heating elements of the heatingassembly, while such heating elements make sliding physical contact withone of the outer exposed surfaces of the sheet.

Using a computer connected to the printer, tests were performed tothermally print various patterns on (in) the multilayer optical film,including the pattern of FIGS. 5 and 5A. In some cases, the multilayeroptical film was fed into the printer in such a way that the heatingelements made contact with the outer exposed surface of the film thatwas nearest the thinner microlayers, i.e., the microlayers whosephysical thickness was about 60 to 70 nm. For convenience, this outersurface of the multilayer optical film is referred to as the “thin-sideouter surface”. In other cases, the multilayer optical film was fed intothe printer in the opposite configuration, such that the heatingelements made contact with the outer exposed surface of the film thatwas nearest the thicker microlayers, i.e., the microlayers whosephysical thickness was about 150 to 170 nm. For convenience, this outersurface of the multilayer optical film is referred to as the “thick-sideouter surface”. Thus, depending on which outer surface of the film wasmade to face and make contact with the heating assembly, two differenttypes of altered regions could be made: “thin-side” altered regions, inwhich the thinner microlayers and ORUs were within the critically heatedvolume (see volume 432 in FIG. 4A) while the thicker microlayers werenot, and “thick-side” altered regions, in which the thicker microlayersand ORUs were within the critically heated volume (see volume 432 inFIG. 4A) while the thinner microlayers were not. Note further that athird type of altered region can be made by combining a thin-sidealtered region with a thick-side altered region, i.e., forming athin-side altered region in at least partial registration with (havingthe same (x,y) coordinates as) a thick-side altered region.

In FIG. 6 we show an Atomic Force Microscopy (AFM) image of an unalteredregion of the patterned reflective polarizer multilayer optical filmdescribed above. In this image, the (unaltered region of the patterned)multilayer optical film 630 can be seen to have a packet 640 ofalternating microlayers, the packet being sandwiched between outer PBLlayers 645, 648. Consistent with the above description of the thicknessgradient, the microlayers, and thus the ORUs, can be seen to be thinnerat one side of the film, near a first outer exposed surface 630 a, andthicker at the opposite side of the film, near a second outer exposedsurface 630 b. FIGS. 6A and 6B show enlarged AFM images of portions ofthe patterned film 630 (in the same unaltered region thereof) near thefirst surface 630 a and the second surface 630 b, respectively. Likenumerals refer to like elements.

In FIG. 7 we show a similar AFM image of the patterned film 630, butthis image is of a “thick-side” altered region of the film that was madeby feeding a sheet of the multilayer optical film through thedirect/thermal transfer printer so that the heating assembly (printhead) of the printer contacted the second outer exposed surface 630 b ofthe film 630. In comparing this image to that of FIG. 6, one can seethat the thicker microlayers, which are adjacent to or near the PBLlayer 648 and outer surface 630 b, have reduced contrast in FIG. 7,while the thinner microlayers near the opposite outer surface 630 a showno loss of contrast. FIG. 7A shows an enlarged AFM image of the portionof the patterned film (in the same thick-side altered region thereof)near the second surface 630 b. The reduced contrast between themicrolayers near the second surface 630 b is also visible in FIG. 7A.The packet of microlayers in this altered region is labeled 640′ todistinguish it from the packet 640 in the unaltered region of the film.Without wishing to be bound by theory, the reduced contrast of thethicker microlayers is believed to be indicative of a change incrystallinity of at least the microlayers that were initiallybirefringent, i.e., a relaxation or reduction in the birefringence ofsuch layers. For the particular reflective polarizer multilayer opticalfilm that was made and tested, such a reduction of birefringence wouldresult in a reduced block-axis refractive index difference betweenmicrolayers, which in turn would result in reduced reflectivity of theblock-axis reflection band for the interfaces associated with thosemicrolayers.

Such an effect was in fact observed when the transmission spectrum ofthe thick-side altered region of the film was measured using aspectrophotometer. In FIG. 7B, curve 702 is the measured normalincidence transmission spectrum for light polarized along the blockaxis, for an unaltered region of the film 630. Consistent with the abovedescription, this curve reveals a reflection band (a band of highreflectivity and low transmission) having a left band edge at about 400nm and a right band edge at about 1000 nm. Curve 704 is the measurednormal incidence transmission spectrum for light polarized along theblock axis, for a thick-side altered region of the film 630. By usingthe curve 702 as a reference and comparing the curve 704 to it, we see asignificant loss in reflectivity, and increase in transmission, at thelonger wavelength side of the reflection band, but little or no loss inreflectivity at the shorter wavelength side of the reflection band. Theloss in reflectivity at the longer wavelengths is indicative of areduction in the block-axis refractive index difference between thethicker microlayers in the microlayer packet. However, the layerstructure of most of the affected thicker microlayers is preserved inthe thick-side altered region, since most of these microlayers can stillbe distinguished from each other in FIGS. 7 and 7A.

Visually, the pattern formed by the thick-side altered regions wascovert, i.e., it was not readily discernible in ordinary unpolarizedambient light. In order to more easily discern the pattern, a polarizerwas used to isolate light polarized along the block axis, and the filmwas viewed (in transmission) at a glancing angle rather than at normalincidence so that the reflection band shifted to shorter wavelengths.Under these conditions, the thick-side altered regions appeared asred-colored regions amidst more neutral gray-colored unaltered regions.Surface distortion was observed in the patterned film, which caused someamount of light scatter.

In FIG. 8 we show another AFM image of the patterned film 630, but thisimage is of a “thin-side” altered region of the film that was made byfeeding a sheet of the multilayer optical film through thedirect/thermal transfer printer so that the heating assembly (printhead) of the printer contacted the first outer exposed surface 630 a ofthe film 630. In comparing this image to that of FIG. 6, one can seethat the thinner microlayers, which are adjacent to or near the PBLlayer 645 and outer surface 630 a, have reduced contrast in FIG. 8,while the thicker microlayers near the opposite outer surface 630 b showno loss of contrast. FIG. 8A shows an enlarged AFM image of the portionof the patterned film (in the same thin-side altered region thereof)near the second surface 630 b. The reduced contrast between themicrolayers near the first surface 630 a is also visible in FIG. 8A. Thepacket of microlayers in this altered region is labeled 640″ todistinguish it from the packet 640 in the unaltered region of the film,and from the packet 640′ in the thick-side altered region of the film.Without wishing to be bound by theory, the reduced contrast of thethinner microlayers is believed to be indicative of a change incrystallinity of at least the microlayers that were initiallybirefringent, i.e., a relaxation or reduction in the birefringence ofsuch layers. For the particular reflective polarizer multilayer opticalfilm that was made and tested, such a reduction of birefringence wouldresult in a reduced block-axis refractive index difference betweenmicrolayers, which in turn would result in reduced reflectivity of theblock-axis reflection band for the interfaces associated with thosemicrolayers.

Such an effect was in fact observed when the transmission spectrum ofthe thin-side altered region of the film was measured using aspectrophotometer. In FIG. 8B, curve 802 is the same as curve 702 inFIG. 7B, i.e., it is the measured normal incidence transmission spectrumfor light polarized along the block axis, for an unaltered region of thefilm 630. Curve 804 is the measured normal incidence transmissionspectrum for light polarized along the block axis, for a thin-sidealtered region of the film 630. By using the curve 802 as a referenceand comparing the curve 804 to it, we see a significant loss inreflectivity, and increase in transmission, at the shorter wavelengthside of the reflection band, but little or no loss in reflectivity atthe longer wavelength side of the reflection band. The loss inreflectivity at the shorter wavelengths is indicative of a reduction inthe block-axis refractive index difference between the thinnermicrolayers in the microlayer packet. However, the layer structure ofmost of the affected thinner microlayers is preserved in the thin-sidealtered region, since most of these microlayers can still bedistinguished from each other in FIGS. 8 and 8A.

Visually, the pattern formed by the thin-side altered regions was faintbut noticeable (discernible) in ordinary unpolarized ambient light. Whenviewing the film in transmission at normal incidence, the thin-sidealtered regions appeared light blue admidst more neutral gray-coloredunaltered regions. The thin-side altered regions appeared darker blue,and the pattern became even more noticeable, when a polarizer was usedto isolate light polarized along the block axis. The thin-side alteredregions became a lighter or duller blue, and the pattern became lessdiscernible, when the film was viewed in transmission at a glancingangle rather than at normal incidence, which caused the reflection bandto shift to shorter wavelengths. Surface distortion was observed in thepatterned film, which caused some amount of light scatter.

The transmission measurements of FIGS. 7B and 8B were taken on thepatterned films. Thus, the reduced reflectivity and increasedtransmission (at the longer wavelengths in FIG. 7B, and at the shorterwavelengths in FIG. 8B) can also be understood as a measure of therelative area within the patterned zone that is fully affected withcomplete loss of birefringence.

In view of the surface distortion and light scatter observed in thepatterned films, attempts were made to apply a hard coat to the outsideof the multilayer optical film before thermally marking the film, toinvestigate the ability of the hard coat to act as a stabilizinginfluence on the layers of the film, without unduly inhibiting thethermal marking. A series of experiments was done to examine the effectsof such a hard coat, which we also refer to as a stabilizing layer.

The hard coat or stabilizing layer was coated onto one side of thereflective polarizer multilayer optical film described above. Inparticular, a UV-curable composition was coated in a layer of uniformthickness onto the outer exposed surface of the multilayer optical filmon the side of the thinner microlayers, e.g., onto the outer exposedsurface 630 a in FIGS. 6 and 6A. The composition was 3M™ 906 AbrasionResistant Coating (48-53 parts isopropyl alcohol, 13-18 partsmethacrylic-functionalized silica, 12-17 parts pentaerythritoltetraacrylate, 5-7 parts other acrylic esters, 3-6 parts pentaerythritoltriacrylate, 2-5 parts N,N-dimethylacrylamide, 1-2 parts1-hydroxycyclohexyl phenyl ketone (IRGACURE 184), <1 part bis(1, 2,2,6,6-pentamethyl-4-piperidinyl) sebacate), which is a curable acrylic resinavailable from 3M Company, St. Paul, Minn. The composition before andafter curing thus comprises a thermoset material. After coating andcuring the composition, the resulting multilayer optical film had a newouter exposed surface on the side of the thinner microlayers, this outersurface corresponding to the outer surface of the newly curedstabilizing layer (hard coat).

The (coated) multilayer optical film was then fed through thedirect/thermal transfer printer in the same manner as described above,with the film oriented so that the outer surface of the stabilizinglayer made sliding physical contact with the heating assembly (printhead) of the printer. In this orientation, the heating assembly wasclosest to the thinner microlayers of the microlayer packet, separatedfrom them by only the stabilizing layer and the 1-micron thick PBL. Justas before, the printer was configured in the direct writing mode toapply maximum thermal power to the sheeting. A print pattern wasselected that included a large enough area so that the spectraltransmission of the altered region of the film could be measured with aspectrophotometer.

This procedure was repeated for a number of different thicknesses of thestabilizing layer, and the results are shown in FIG. 9. That figureshows the measured spectral transmission of the altered region (thethin-side altered region) of the (coated) multilayer optical filmsamples that were made and to which heat was delivered, the film samplesbeing substantially the same except that the (cured) stabilizing layerwas of different thicknesses for the different samples. The spectraltransmission was measured using normally incident light polarizedparallel to the block axis. Curve 902 is for a sample that had nostabilizing layer (stabilizing layer thickness=0). Curve 904 is for asample whose stabilizing layer had a thickness of 2.4 microns. Curve 906is for a sample whose stabilizing layer had a thickness of 3.0 microns.Curve 908 is for a sample whose stabilizing layer had a thickness of 4.8microns. Curve 910 is for a sample whose stabilizing layer had athickness of 6.4 microns. Curve 912 is for a sample whose stabilizinglayer had a thickness of 7.7 microns. Curve 914 is for a sample whosestabilizing layer had a thickness of 4.2 microns. Curve 916 is for asample whose stabilizing layer had a thickness of 7.7 microns. Curve 918is for a sample whose stabilizing layer had a thickness of 8.8 microns.Curve 920 is for a sample whose stabilizing layer had a thickness of11.7 microns. Curve 922 is provided for reference purposes. This curveis not of an altered region, but of an unaltered region of themultilayer optical film.

Comparison of the curves in FIG. 9 allows us to quantify the effect ofthe stabilizing layer, and in particular the effect of its thickness onhow much of a change the marking process has on the opticalcharacteristics (including the reflective and transmissivecharacteristics) of the multilayer optical film. Keeping in mind thatthe transmission and reflection characteristics of this multilayeroptical film are substantially complementary, i.e., T+R≈100%, curve 922shows how the reflection band of the original (non-altered ornon-marked) multilayer optical film has a left band edge near 400 nm,and extends to longer wavelengths beyond the 800 nm boundary of thegraph. Curve 902 shows how, with no stabilizing layer, the shortwavelength side of the reflection band in the altered region diminishesor degrades more than any of the other curves. Curves 904 through 920show a general trend wherein the greater the thickness of thestabilizing layer, the less the degradation or change of the reflectionband, relative to the original reflection band of curve 922, and viceversa.

The amount of change of the reflective characteristic as a function ofthe thickness of the stabilizing layer can be further quantified byplotting the left band edge of each curve in FIG. 9 against thethickness of the stabilizing layer for that curve. The left band edgecan be taken to be, for example, the wavelength at which thereflectivity falls to one-half of its maximum in-band value. By plottingthe resulting points, the curve 1002 of FIG. 10 is obtained. Thehorizontal dashed line at a wavelength of 420 nm is the left band edgecalculated in this manner for the curve 922, i.e., a baseline from whichany deviations are made. The curve 1002 reveals that for a stabilizinglayer thickness of roughly 10 microns or greater, little or no change inthe reflective characteristic of the film, as indicated by thewavelength of the left band edge, is obtained. The curve 1002 alsoreveals that a maximum change in the reflective characteristic isobtained by a minimal thickness of the stabilizing layer thickness, e.g.about 1 micron or less, although thicknesses less than about 10 micronsproduce some change.

The results of FIG. 10 do not take into account light scatter caused bysurface distortion. Scattering due to such surface distortion isdepicted schematically in FIG. 11. In that figure, a multilayer opticalfilm 1130 has outer exposed surfaces 1130 a, 1130 b, a packet 1140 ofmicrolayers, and a stabilizing layer 1145 attached to and covering thepacket 1140. A volume 1132 represents the portion of the film 1130 whatwas critically heated sufficiently to reduce the birefringence ofbirefringent microlayers within such volume. Thus, the film 1130 is amarked or patterned film, and the regions 1134 are unaltered regions ofthe film, while the region 1136 is an altered region of the filmcorresponding to the volume 1132. Surface distortion is represented byundulating or roughened interfaces between microlayers in the volume1132, and an undulating or roughened portion of the outer exposedsurface 1130 a. These roughened surfaces divert light to producescattered light 1101. The amount of scattered light can be quantified bymeasuring the haze of the film in the region of interest, e.g., in thealtered region 1136.

To investigate the effect of the stabilizing layer on the surfacedistortion and haze, coated multilayer optical films were made in thesame manner as described above in connection with FIGS. 9 and 10, butwith a series of different thicknesses of the cured acrylate materialwhich formed the stabilizing layer. These films were patterned with thesame model 110XIIIIPLUS Industrial Printer made by Zebra Technologiesreferred to above, with the film oriented so that the heating assemblyof the printer made sliding contact with the outer surface of the filmcorresponding to the outer surface of the stabilizing layer, which wasagain proximate the thinner microlayers of the microlayer packet,separated from them by only the 1-micron thick PBL. Although the printerwas configured in the direct writing mode as before, in this series oftests, the thermal power of the heating assembly was adjusted for eachof the film samples so that, after delivery of the heat to pattern ormark the film, the reflection band for the block state polarization inthe altered region had a left band edge at about 520 nm, which wasconfirmed by measurement with a spectrophotometer. In addition to thismeasurement, the altered region of the film was also measured for hazeusing a Haze Gard Plus instrument available from BYK-Gardiner, SilverSprings, Md.

The results of these measurements are shown in the combination graph ofFIG. 12. In that figure, curve 1202 shows the measured wavelength of theleft band edge (LBE) (see left vertical scale) as a function of the(physical) thickness of the stabilizing layer, for the altered region ofeach tested film sample. Curve 1204 shows the measured haze (see rightvertical scale) for the same altered regions as a function of the samethickness of the stabilizing layer, for the tested film samples.Inspection of the curve 1204 reveals that when the thickness of thestabilizing layer about 0.5 microns or less, the haze of the alteredregion increases rapidly. Thus, by providing a stabilizing layer that isgreater than 0.5 microns thick, the optical haze of the multilayeroptical film due to layer distortion in the altered regions can be nomore than 20%, or no more than 10%.

Additional features that can be provided in the disclosed patternedmultilayer optical films include a layer or layers that can be added tothe film to render one or both sides of the multilayer optical film, orportions thereof, non-writable with regard to the disclosed thermalmarking technique. Examples of such features are shown in FIG. 13A andfollowing. In brief, a thermal buffer layer is coated onto all or atleast a portion of at least one of the major surfaces of the film. Thefilm may or may not have been thermally patterned or marked on the sideof the major surface(s) at issue before the coating of the thermalbuffer layer. The thermal buffer layer has a composition and thicknessthat prevent changing the reflective characteristic of the multilayeroptical film through the thermal buffer layer using the thermal markingprocedures described herein. In some cases the thermal buffer layer maycompletely cover one side of the film in a layer of uniform thickness.In such cases, that side of the film is rendered substantially entirelynon-writable. In other cases the thermal buffer layer may effectivelycover only portions of the side of the film, e.g., the layer may beentirely absent in some portions, or may it may be substantially thinnerin some places than in other places. In such cases, portions of the filmfor which the thermal buffer layer is thick are rendered non-writable,while portions of the film for which the thermal buffer layer is thinner(including substantially absent) continue to be writable, i.e., theirreflective characteristic may be changed by the disclosed thermalmarking technique.

The thermal buffer layer may be made of any suitable composition, e.g.,an organic and/or inorganic material. In order to minimize damage andcontamination of the thermally printed image, the layer should have ahigh thermal resistance. Typically, the layer should not visibly distortor chemically decompose at temperatures reached at the surface of thethermal buffer layer during the thermal printing process. Theseproperties may be readily provided by polymeric film (thermoplastic orthermoset layers), inorganic layers (e.g., sol-gel deposited layers,vapor-deposited layers of inorganic oxides such as silica, titania,etc., including metal oxides), and organic/inorganic composite layers(thermoplastic or thermoset layers). Organic materials suitable for usein the thermal buffer layer include both thermoset (crosslinked) andthermoplastic materials. In both cases, the material chosen for thethermal buffer layer should be film forming and should remainsubstantially intact during the printing process. This can beaccomplished by the proper selection of materials based on their thermaland/or mechanical properties. In typical embodiments, the thermal bufferlayer is substantially transparent to visible light, but it may bytinted or colored e.g. with one or more pigments, dyes, and/or othercolorants if desired. The minimum thickness of the thermal buffer layerthat would render a side of the multilayer optical film non-writable maydepend to some extent on design details of the heating element(s) andthe multilayer optical film, but in many cases a thickness of at least 5microns or at least 10 microns is sufficient to render the underlyingmultilayer optical film non-writable. Keeping the thickness of thethermal buffer layer within a range from 5 to 50 microns, or from 10 to100 microns, may render the pertinent portion of the film non-writablewhile also maintaining a good mechanical flexibility of the coated film.

In FIG. 13A, a multilayer optical film 1330 of the type described hereinis shown schematically. The film 1330 has outer exposed surfaces 1330 a,1330 b, a packet 1340 of microlayers, and a stabilizing layer 1345attached to and covering the packet 1340. The film 1330 also has anotherstabilizing layer 1348 disposed opposite the stabilizing layer 1345, thelayer 1348 also attached to and covering the packet 1340. As shown, thefilm 1330 is in an initial or pre-marked state in FIG. 13A.

As shown in FIG. 13B, the film 1330 may then be marked using the thermalmarking procedure discussed above in connection with FIGS. 1, 2, and 4,to provide critically heated volumes 1332 and the associated alteredregions 1336, amidst unaltered regions 1334. The film is labeled 1330′,and the packet of microlayers is labeled 1340′, to reflect the localizedchanges in the packet of microlayers relative to the original packet1340. The volumes 1332 and regions 1334, 1336 may be the same as orsimilar to corresponding volumes and regions described elsewhere hereinto provide a desired pattern of markings in the film.

Following this marking procedure, as shown in FIG. 13C, the marked film1330′ may then be coated with a suitable composition to provide athermal buffer layer 1346 on one side of the film, which layer 1346attaches to and covers some or substantially all of the stabilizinglayer 1345. Due to the presence of the thermal buffer layer 1346, themarked multilayer optical film is relabeled 1330″. As noted above, thethermal buffer layer 1346 has a thickness that is sufficient to renderthe underlying multilayer optical film non-writable. That is, if a userattempts to thermally treat a piece of the film 1330″ in the mannerdescribed above in connection with FIG. 4A, e.g. by feeding the (coated)multilayer optical film 1330″ through the direct/thermal transferprinter described above, with the film oriented so that the newly formedouter surface 1330 a′ of the film makes sliding physical contact withthe heating assembly (print head) of the printer, insufficient heat isdelivered to the film to provide any additional altered regions, and thereflection characteristic of the film at any given position remainssubstantially unchanged. The pattern of unaltered regions 1334 andaltered regions 1336 that were formed before application of the thermalbuffer layer 1346 remain, but no additional altered regions can beformed from the upper side of the film 1330″ as seen in FIG. 13C. (Note,however, that in some cases the film 1330″ may be patterned using thedisclosed thermal marking technique by physically contacting the heaterelement to the lower side of the film, i.e., to the outer exposedsurface 1330 b, which is shown in FIG. 13C to include no thermal bufferlayer. To prevent this capability, another thermal buffer layer, whichmay otherwise be the same as or similar to the thermal buffer layer1346, can be coated onto all or a portion of the lower side of the filmat the outer exposed surface 1330 b.)

A sequence of figures showing an example of a patterned thermal bufferlayer are shown in FIGS. 14A through 14D. In FIG. 14A, a multilayeroptical film 1430 of the type described herein is shown schematically.The film 1430 has outer exposed surfaces 1430 a, 1430 b, a packet 1440of microlayers, and a stabilizing layer 1445 attached to and coveringthe packet 1440. The film 1430 also has another stabilizing layer 1448disposed opposite the stabilizing layer 1445, the layer 1448 alsoattached to and covering the packet 1440. As shown, the film 1430 is inan initial or pre-marked state in FIG. 14A.

The film 1430 may then be marked, as shown in FIG. 14B, using thethermal marking procedure discussed above in connection with FIGS. 1, 2,and 4, to provide critically heated volumes 1432 and the associatedaltered regions 1436, amidst unaltered regions 1434. The film is labeled1430′, and the packet of microlayers is labeled 1440′, to reflect thelocalized changes in the packet of microlayers relative to the originalpacket 1440. The volumes 1432 and regions 1434, 1436 may be the same asor similar to corresponding volumes and regions described elsewhereherein to provide a desired pattern of markings in the film.

Following this marking procedure, as shown in FIG. 14C, the marked film1430′ may then be coated with a suitable composition to provide athermal buffer layer 1446 on one side of the film, which layer 1446attaches to and covers only a portion of the stabilizing layer 1445, andis absent (or substantially absent) from other portions of the layer1445. Due to the presence of the thermal buffer layer 1446, the markedmultilayer optical film is relabeled 1430″. As noted above, the thermalbuffer layer 1446 has a thickness that is sufficient to render theunderlying multilayer optical film non-writable. That is, if a userattempts to thermally treat a piece of the film 1430″ in the mannerdescribed above in connection with FIG. 4A, e.g. by feeding the (coated)multilayer optical film 1430″ through the direct/thermal transferprinter described above, with the film oriented so that the newly formedouter surface 1430 a′ of the film (a portion of which coincides with theoriginal outer exposed surface 1430 a and a portion of which coincideswith the outer exposed surface of the thermal buffer layer 1446) makessliding physical contact with the heating assembly (print head) of theprinter, insufficient heat is delivered to the film to provide anyadditional altered regions at positions covered by the layer 1446.However, in such a thermal marking procedure, as shown in FIG. 14D, dueto the absence of the thermal buffer layer in remaining portions of thefilm, sufficient heat can be delivered to such remaining portions toprovide additional critically heated volumes 1432′, with correspondingadditional altered regions 1436′ having appropriately modifiedreflection characteristics. Due to the presence of the additionalcritically heated volumes 1432′ and altered regions 1432′, the film islabeled 1430″ and the packet of microlayers is labeled 1440″. The films1430″ and 1430′″ thus each have portions that are non-writable from theupper side of the film, as well as writable portions, due to thepatterned nature of the thermal buffer layer 1446.

Similar to the discussion of FIG. 13C above, in some cases the film1430′ (or any of its predecessor films in FIGS. 14A through 14C) may bepatterned using the disclosed thermal marking technique by physicallycontacting the heater element to the lower side of the film, i.e., tothe outer exposed surface 1430 b, which is shown in FIG. 14D to includeno thermal buffer layer. To prevent this capability, another thermalbuffer layer, which may otherwise be the same as or similar to thethermal buffer layer 1446, can be coated onto all or a portion of thelower side of the film at the outer exposed surface 1430 b.)

FIG. 15 is a schematic top or plan view of a multilayer optical filmthat shows patterning associated with thermal marking, as well aspatterning associated with a patterned thermal buffer layer. Themultilayer optical film 1530, or portions thereof, may be the same as orsimilar to other multilayer optical films described herein. The film1530 has been patterned using the disclosed thermal marking technique toprovide altered regions 1536 and unaltered regions 1534. Some of thealtered regions are shown to form alphanumeric text, while others areshown to form a 2-dimensional bar code pattern, such as a QR code. Theseare merely examples, and other desired patterns are also contemplated.After this patterning is complete, a thermal buffer layer is thenselectively coated in a patterned fashion in a region 1540, which region1540 then becomes a non-writable region to prevent unauthorized usersfrom adding or modifying any of the patterning in that region, e.g. as asecurity or anti-counterfeiting measure. The thermal buffer layer ishowever not coated in another region 1542, which may be complementary tothe writable region 1540. This allows users to pattern the film 1530using the disclosed thermal marking technique in the region 1542, whichmay be considered to be a writable region of the film 1530. A user may,for example, print a halftone image of a person's likeness, or any otherdesired pattern, in the writable region 1542 using the disclosed thermalmarking technique. Note that in this embodiment, as well as theembodiments of FIGS. 13C, 14C, and 14D, the non-writable zone at leastpartially overlaps with the altered regions.

Unless otherwise indicated, all numbers expressing quantities,measurement of properties, and so forth used in the specification andclaims are to be understood as being modified by the term “about”.Accordingly, unless indicated to the contrary, the numerical parametersset forth in the specification and claims are approximations that canvary depending on the desired properties sought to be obtained by thoseskilled in the art utilizing the teachings of the present application.Not as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. Notwithstandingthat the numerical ranges and parameters setting forth the broad scopeof the invention are approximations, to the extent any numerical valuesare set forth in specific examples described herein, they are reportedas precisely as reasonably possible. Any numerical value, however, maywell contain errors associated with testing or measurement limitations.

Any direction referred to herein, such as “top,” “bottom,” “left,”“right,” “upper,” “lower,” “above,” “below,” and other directions andorientations are used for convenience in reference to the figures andare not to be limiting of an actual device, article, or system or itsuse. The devices, articles, and systems described herein may be used ina variety of directions and orientations.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the spirit and scopeof this invention, and it should be understood that this invention isnot limited to the illustrative embodiments set forth herein. The readershould assume that features of one disclosed embodiment can also beapplied to all other disclosed embodiments unless otherwise indicated.It should also be understood that all U.S. patents, patent applicationpublications, and other patent and non-patent documents referred toherein are incorporated by reference, to the extent they do notcontradict the foregoing disclosure.

This document discloses numerous embodiments, including but not limitedto the following:

Item 1 is a method of making a patterned multilayer optical film,comprising:

-   -   providing a multilayer optical film having an outer exposed        surface and a packet of microlayers arranged to selectively        reflect light by constructive or destructive interference to        provide a first reflective characteristic, at least some of the        microlayers being birefringent, the multilayer optical film also        including a stabilizing layer attached to and covering the        packet of microlayers proximate the outer exposed surface;    -   physically contacting the multilayer optical film with one or        more heating elements to deliver heat at one or more altered        regions of the film to the packet of microlayers through the        stabilizing layer by thermal conduction, such that the first        reflective characteristic changes to an altered reflective        characteristic in the altered regions to pattern the multilayer        optical film, the stabilizing layer being tailored to provide        sufficient heat conduction to allow heat from the heating        elements to change the birefringence of the birefringent        microlayers disposed near the outer exposed surface in the        altered regions, while also providing sufficient mechanical        support to avoid substantial layer distortion of the microlayers        near the outer exposed surface in the altered regions.        Item 2 is the method of item 1, wherein the stabilizing layer is        tailored such that after the physically contacting, the optical        haze of the optical film due to layer distortion in the altered        regions is no more than 20%, or no more than 10%.        Item 3 is the method of item 1, wherein after the physically        contacting, the patterned multilayer optical film has one or        more unaltered regions in addition to the one or more altered        regions, and wherein a group of first microlayers from the        birefringent microlayers have respective refractive indices that        are substantially unchanged in the altered regions relative to        the unaltered regions, and a group of second microlayers from        the birefringent microlayers have respective refractive indices        that are substantially changed in the altered regions relative        to the unaltered regions, the group of second microlayers being        closer than the group of first microlayers to the outer exposed        surface.        Item 4 is the method of item 1, wherein the physical contact is        a sliding contact.        Item 5 is the method of item 4, wherein the multilayer optical        film further includes a lubricant layer comprising a non-polymer        lubricant material covering the stabilizing layer.        Item 6 is the method of item 4, wherein the one or more heating        elements includes a set of individually addressable heating        elements, the method further comprising:    -   providing an extended heating assembly, the heating assembly        including the individually addressable heating elements;    -   wherein the physically contacting includes moving the multilayer        optical film in relation to the extended heating assembly such        that the outer exposed surface of the multilayer optical film        makes sliding contact with the extended heating assembly, and        selectively heating the heating elements during the moving to        provide the one or more altered regions.        Item 7 is the method of item 1, further comprising:    -   after the physically contacting is carried out to provide the        patterned multilayer optical film, coating at least a first zone        of the patterned multilayer optical film at its outer exposed        surface with a thermal buffer layer, the thermal buffer layer        forming a new outer exposed surface to provide a coated        patterned multilayer optical film.        Item 8 is the method of item 7, wherein the thermal buffer layer        has a sufficient thickness so that the one or more heating        elements provide little or no change in the first reflective        characteristic in the first zone of the multilayer optical film        upon physically contacting the new outer exposed surface at such        first portion with the one or more heating elements, such that        the first zone is a non-writable zone.        Item 9 is the method of item 1, wherein the outer exposed        surface is a first outer exposed surface and the multilayer        optical film further comprises a second outer exposed surface        opposite the first outer exposed surface, and wherein the        physically contacting comprises physically contacting the first        outer exposed surface with the one or more heating elements to        provide one or more first altered regions, and the physically        contacting further comprises physically contacting the second        outer exposed surface with the one or more heating elements to        provide one or more second altered regions.        Item 10 is the method of item 9, wherein the packet of        microlayers are characterized by a layer thickness gradient such        that microlayers proximate the first outer exposed surface are        thicker than microlayers proximate the second outer exposed        surface, such that the one or more first altered regions have a        first altered reflective characteristic and the one or more        second altered regions have a second altered reflective        characteristic different from the first altered reflective        characteristic.        Item 11 is a patterned multilayer optical film having an outer        exposed surface, comprising:    -   a packet of microlayers arranged to selectively reflect light by        constructive or destructive interference to provide a first        reflective characteristic, the microlayers comprising        thermoplastic materials; and    -   a stabilizing layer attached to and covering the packet of        microlayers proximate the outer exposed surface, the stabilizing        layer comprising a thermoset material;    -   wherein the packet of microlayers is selectively altered in a        pattern to provide the first reflective characteristic in one or        more unaltered regions and a second reflective characteristic,        different from the first reflective characteristic, in one or        more altered regions;    -   wherein the packet of microlayers includes a first and second        group of microlayers each having a birefringence in the        unaltered regions, and wherein the first group of microlayers        substantially maintain the birefringence in the altered regions,        and the second group of microlayers have a changed birefringence        in the altered regions relative to the unaltered regions, the        group of second microlayers being closer than the group of first        microlayers to the outer exposed surface; and wherein the one or        more altered regions have an optical haze of no more than 20%,        or no more than 10%.        Item 12 is the film of item 11, wherein the stabilizing layer        has a physical thickness in a range from greater than 0.5        microns to less than 10 microns.        Item 13 is the film of item 11, wherein the stabilizing layer is        a hard coat layer.        Item 14 is the film of item 11, further comprising a lubricant        layer attached to and covering the stabilizing layer, the        lubricant layer comprising a non-polymer lubricant material        comprising wax.        Item 15 is the film of item 11, further comprising a thermal        buffer layer at least partially covering the stabilizing layer,        the thermal buffer layer being effective to inhibit heat-induced        birefringence reduction of the second group of microlayers in        one or more zones of the film in which the thermal buffer layer        covers the stabilizing layer, such zones referred to as        non-writable zones.        Item 16 is the film of item 15, wherein the thermal buffer layer        covers substantially an entire major surface of the stabilizing        layer, such that substantially all of the film is rendered        non-writable.        Item 17 is the film of item 15, wherein the thermal buffer layer        is substantially absent from one or more zones of the film, such        zones referred to as writable zones, such that the film        comprises both writable zones and non-writable zones.        Item 18 is the film of item 17, wherein the one or more        non-writable zones at least partially overlap with the one or        more altered regions.        Item 19 is a multilayer optical film having an outer exposed        surface, comprising:    -   a packet of microlayers arranged to selectively reflect light by        constructive or destructive interference to provide a first        reflective characteristic, the microlayers comprising        thermoplastic materials, at least some of the microlayers being        birefringent; and a stabilizing layer attached to and covering        the packet of microlayers proximate the outer exposed surface;    -   wherein the stabilizing layer comprises a thermoset material and        is tailored to, upon exposure of a region of the film to a        thermal printer at the outer exposed surface, provide sufficient        heat conduction to allow heat from the thermal printer to change        the birefringence of the birefringent microlayers disposed near        the outer exposed surface in such exposed region, while also        providing sufficient mechanical support to inhibit distortion of        the microlayers near the outer exposed surface in such exposed        region, the changed birefringence associated with an altered        reflective characteristic for the packet of microlayers        different from the first reflective characteristic.        Item 20 is the film of item 19, wherein the stabilizing layer        has a physical thickness in a range from greater than 0.5        microns to less than 10 microns.        Item 21 is the film of item 19, wherein the outer exposed        surface is a surface of the stabilizing layer.        Item 22 is the film of item 19, wherein the stabilizing layer        provides sufficient mechanical support such that, upon the        exposure of the region of the film to the thermal printer at the        outer exposed surface, an optical haze of the optical film due        to layer distortion in such exposed region is no more than 20%,        or no more than 10%.        Item 23 is the film of item 19, the film further comprising a        lubricant layer covering the stabilizing layer, the lubricant        layer comprising a non-polymer lubricant material comprising        wax.        Item 24 is the film of item 19, wherein the stabilizing layer is        a hard coat layer.        Item 25 is the film of item 19, further comprising a thermal        buffer layer partially covering the stabilizing layer, the        thermal buffer layer being patterned to have a variable        thickness to define one or more writable zones and one or more        non-writable zones of the film.        Item 26 is the film of item 25, wherein the thermal buffer layer        has a physical thickness in the one or more non-writable zones,        the physical thickness being at least 5 microns.        Item 27 is the film of item 26, wherein the thermal buffer layer        is substantially absent from the one or more writable zones.

What is claimed is:
 1. A patterned multilayer optical film having anouter exposed surface, comprising: a packet of microlayers arranged toselectively reflect light by constructive or destructive interference toprovide a first reflective characteristic, the microlayers comprisingthermoplastic materials; and a stabilizing layer attached to andcovering the packet of microlayers proximate the outer exposed surface,the stabilizing layer comprising a thermoset material; wherein thepacket of microlayers is selectively altered in a pattern to provide thefirst reflective characteristic in one or more unaltered regions and asecond reflective characteristic, different from the first reflectivecharacteristic, in one or more altered regions; wherein the packet ofmicrolayers includes a first and second group of microlayers each havinga birefringence in the unaltered regions, and wherein the first group ofmicrolayers substantially maintain the birefringence in the alteredregions, and the second group of microlayers have a changedbirefringence in the altered regions relative to the unaltered regions,the group of second microlayers being closer than the group of firstmicrolayers to the outer exposed surface; wherein the one or morealtered regions have an optical haze of no more than 20%; and whereinthe packet of microlayers is configured such that the multilayer opticalfilm is a polarizer or mirror.
 2. The patterned multilayer optical filmof claim 1, wherein the packet of microlayers is configured such thatthe multilayer optical film is a polarizer.
 3. The patterned multilayeroptical film of claim 1, wherein the packet of microlayers is configuredsuch that the multilayer optical film is a mirror.
 4. The patternedmultilayer optical film of claim 1, wherein the packet of microlayers isconfigured to reflect in a visible or near infrared portions of anelectromagnetic spectrum.
 5. A multilayer optical film having an outerexposed surface, comprising: a packet of microlayers arranged toselectively reflect light by constructive or destructive interference toprovide a first reflective characteristic, the microlayers comprisingthermoplastic materials, at least some of the microlayers beingbirefringent; and a stabilizing layer attached to and covering thepacket of microlayers proximate the outer exposed surface; wherein thestabilizing layer comprises a thermoset material and is tailored to,upon exposure of a region of the film to a thermal printer at the outerexposed surface, provide sufficient heat conduction to allow heat fromthe thermal printer to change the birefringence of the birefringentmicrolayers disposed near the outer exposed surface in such exposedregion, while also providing sufficient mechanical support to inhibitdistortion of the microlayers near the outer exposed surface in suchexposed region, the changed birefringence associated with an alteredreflective characteristic for the packet of microlayers different fromthe first reflective characteristic; wherein the packet of microlayersis configured such that the multilayer optical film is a polarizer ormirror.
 6. The multilayer optical film of claim 5, wherein the packet ofmicrolayers is configured such that the multilayer optical film is apolarizer.
 7. The multilayer optical film of claim 5, wherein the packetof microlayers is configured such that the multilayer optical film is amirror.
 8. The multilayer optical film of claim 5, wherein the packet ofmicrolayers is configured to reflect in a visible or near infraredportions of an electromagnetic spectrum.